[mips] Reserve address spaces 1-255 for software use.

[dsanders]

Summary

And define them to have noop casts with address spaces 0-255.

[theraven]

I only just spotted this. Given that you know that we're using AS 200 for a hardware AS, it would have been nice to have been cc'd on this as a heads up.

[dsanders]

Sorry, I didn't know you were using AS 200. I can special case that or reduce the reserved range if you want. There wasn't a good reason for picking <256 in particular.

[theraven]

We can maintain a patch locally, but please keep it in mind for future changes.

[dsanders]

Ok. Sorry for causing problems in your port.

lib/Target/Mips/MipsISelLowering.h

struct LTStr {
       }
     };
 
     void HandleByVal(CCState *, unsigned &, unsigned) const override;
 
     unsigned getRegisterByName(const char* RegName, EVT VT,
                                SelectionDAG &DAG) const override;
 
+    /// Returns true if a cast between SrcAS and DestAS is a noop.
+    bool isNoopAddrSpaceCast(unsigned SrcAS, unsigned DestAS) const override {
+      // Mips doesn't have any special address spaces so we just reserve
+      // the first 256 for software use (e.g. OpenCL) and treat casts
+      // between them as noops.
+      return SrcAS < 256 && DestAS < 256;
+    }
+
   protected:
     SDValue getGlobalReg(SelectionDAG &DAG, EVT Ty) const;
 
     // This method creates the following nodes, which are necessary for
     // computing a local symbol's address:
     //
     // (add (load (wrapper $gp, %got(sym)), %lo(sym))
     template <class NodeTy>

test/CodeGen/Mips/llvm-ir/addrspacecast.ll

+; RUN: llc < %s -march=mips -mcpu=mips2 | FileCheck %s -check-prefix=ALL
+
+; Address spaces 1-255 are software defined.
+define i32* @cast(i32 *%arg) {
+  %1 = addrspacecast i32* %arg to i32 addrspace(1)*
+  %2 = addrspacecast i32 addrspace(1)* %1 to i32 addrspace(2)*
+  %3 = addrspacecast i32 addrspace(2)* %2 to i32 addrspace(0)*
+  ret i32* %3
+}
+
+; ALL-LABEL: cast:
+; ALL:           move   $2, $4